Some photographers never ever crop a frame. They see the image as it was taken in the camera as a holy object which can't be sullied by cropping. Others will do whatever they damn well please to the image. I'm somewhere in the middle. Or rather it's more of a split decision.
On film, I don't crop. In fact I scan all the way past the edge of the frame so that the borders are in any prints. I started doing this because that's what Cartier-Bresson did and I wanted to be like Henri. That way the film frame is somehow sacred. I had a discussion with Timothy Greenfield-Sanders once and he's firmly in the never crop camp. That said, he's almost always shooting film and he spends a crazy amount of time getting the images just right in the camera before he ever opens the shutter. He's got to. He's shooting 8x10" or even 11x14" film which costs tens of dollars a frame. Measure twice, cut once. For example, my whole series of images from Japan were shot on a Hasselblad and I made sure I kept the borders in. It's more authentic that way. What the camera saw is what I was looking at through the viewfinder. Very 1 to 1.

Now on the flip side is digital where sometimes I crop a lot. However most of the time I'm shooting with the cropping in mind, and I always crop to standard ratios. For example I'll often shoot 2x3 with the 5D and then crop it to 4x5 to make them look as if I could afford to shoot them on my 4x5 (grin). I'll also sometimes crop square from the digital in order to match the view from my Hasselblad. In fact, if I crop my 35mm lens square, I end up with a field of view that's almost identical to the Hassy with a standard 80mm lens. That's why I bought the 35mm lens actually. Cheesy but true.

There are plenty of other people who do fascinating stuff with odd crops and collages and I'm not saying anything against them. I'm a 63 year old photographer in the body of a 36 year old. I like the way classic things look. I like to pay homage to the history of it all. For all of my back and forth on subjects like this, I do tend to have very ridged rules. Things I will do and things that I won't ever do. It can be confusing at times, but then again, so can being alive. Deal with it.

Update: ONE MORE THING
I was just thinking about another time you may need to crop. If you happen to drop your 50mm lens on the way through security and into a portrait shoot for TIME and have to shoot the whole thing with your 28mm prime because that's the only lens you've got. Luckily the 5D2 has a lot of pixels. Enough to crop a fair bit and still be able to print full page.